Have you tried connecting via ethernet? Go onto the Home Hub page and click on A-Z. Navigate to Wireless Connections Enable or disable. Change the wireless interface type to 802.11 b/g. Try changing the wireless channel on your router by going onto your router's admin pages - channels 1, 6 and 11 are usually the most efficient.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been